Monday, February 3, 2014

The bright and occasional north star, Vega, sits low on the horizon and shimmers across a foggy
Lake McDonald between the dim winter Milky Way (left) and Comet Lovejoy (right). (Click to enlarge.)

On the rare cloudless night, thick blankets of stars warm our winter sky over western Montana - at least for those of us living beyond the shroud of city lights. And from lounging around summertime campfires we remember that the constellations seem to spin circles around the "north star," named Polaris, which though dim is still the brightest star in the Little Dipper constellation.

But Polaris isn't always our north star.

Polaris currently sits almost straight in line with the Earth's north-south axis of rotation. But the Earth isn't perfectly round, and gravity from the sun and moon pulls a little wobble into our spin. This wobble makes that imaginary point in the northern sky (the "north celestial pole") move in a giant circle lasting 25,765 years, a movement called the Earth's "precession." This means that over time the precession changes which star we perceive to be the north star. Several stars (and a whole lot of empty space) happen to fall along this circular path.

When a pinpoint of light arrives at the north celestial pole, and appears in the same place for many human generations, it is quite useful to the people living on this wobbly, spinning top. For our cave-dwelling ancestors 12,000 years ago, it was the bright star Vega that aligned as their north star. And in roughly 12,000 years from now, Vega will once again take its turn as our north star, passing within four degrees of the north celestial pole. Vega is more than twice as massive as our star, the Sun, and it is the second-brightest star visible from the northern hemisphere (after Arcturus).

The star we now know as Thuban served as the north star for the ancient engineers, who used Thuban to help them design and build Stonehenge and the Great Pyramids, roughly 4,000-2,700 years ago. Even though it's a dim star, Thuban sat precisely in the polar position in the year 2,787 B.C. And roughly 21,000 years from now, Thuban will once again reoccupy this prime spot in the night sky.

Our current navigational star, Polaris, is one of the dimmest stars that we can see with the naked eye. Polaris currently sits about 0.7 degrees off the celestial north pole, heading out on its long journey around the night sky. It'll complete its current lap in about 23,000 years. (Polaris is also the current pole star for Saturn, about 6 degrees off center.)

Down in the southern hemisphere, the south celestial pole currently falls in a dark region within the constellation Octans. The nearest stars are very faint, so the folks down under currently don't have a "south star." Just one more reason to spend a little time admiring the dark, star-lit skies over Montana, and anywhere else that the sky is still dark enough to contemplate the stars.